1. Field of the Invention
The present invention relates to an electronically controlled exhaust gas recirculation (EGR) appratus which recirculates a portion of exhaust gas to an intake system of an engine under a predetermined condition to reduce a NO.sub.x content.
2. Description of the Prior Art
An exhaust gas recirculation (EGR) apparatus recirculates a portion of inactive exhaust gas to a intake system and mixes it with intake gas mixture to lower a combustion temperature in order to reduce production of NO.sub.x content in the exhaust gas.
In the ERG, a portion of the exhaust gas is fed from an exhaust manifold to a intake manifold through an EGR valve. With a trend of electronization of controls for an automobile, the electronization of the EGR apparatus has also been desired. In one example, an electronic control is effected by utilizing the EGR valve. In this case, a vacuum is applied to a diaphragm to control the opening of the EGR valve. In order to attain the electronic control by using a conventional EGR valve, a pressure adjusting device having a solenoid valve which is controlled by an electronic circuit is arranged in a path which supplies an intake manifold vacuum or a vacuum from a vacuum source such as a vacuum pump, to control the opening of the EGR valve by the vacuum adjusted by the pressure adjusting device. The intake manifold vacuum of the engine or a vacuum pump driven by the engine is usually used as a vacuum source to the pressure adjusting device.
However, in such a prior art system which used the EGR valve, the opening of the EGR valve changes with a variation of pressure of the vacuum source. As a result, it is not possible to uniquely and exactly control the opening of the EGR valve.